1. Field of the Invention
The present invention concerns a nuclear-medical magnetic resonance atlas as well as a method for generation of such a magnetic resonance atlas. The invention also concerns a method for generation of a nuclear-medical image with such a magnetic resonance atlas.
2. Description of the Prior Art
In addition to computed tomography (CT) and magnetic resonance tomography (MRT), nuclear medicine (NM) is an important modality for imaging diagnostics. NM supplies primary and/or additional diagnostic results and thus forms an important component piece in the overall diagnostic scene. NM uses radioactive isotopes for qualitative or quantitative analysis of, among other things, diffusion and metabolic processes. A specific enrichment of the various isotopes in the organs enables a sensitive functional diagnostic of nearly all organ systems. Positron emission tomography (PET) or single photon emission computer tomography (SPECT) are examples of modem nuclear-medical modalities for medical imaging.
In NM imaging, among other things information regarding scattering and attenuation of the detected radiation or particles must be accounted for in the evaluation of NM data. The required information can be acquired, for example, with a test measurement. An alternative is to draw the information from a registration of the NM data with an NM atlas; this means that the NM data are superimposed with an NM atlas image containing, for example, the attenuation coefficients.
The use of an anatomical NM atlas (more precisely a PET atlas) for scatter and attenuation correction of PET images is known, for example, from U.S. Pat. No. 6,740,883. For this purpose, a three-dimensional computer model with a PET image is superimposed and aligned as a PET atlas. The computer model represents the density distribution within the acquisition region of interest and is generated by averaging of existing transmission or CT images of a number of patients. In one embodiment, the computer model includes a functional component that simulates a PET or SPECT image of the acquisition region and an anatomical component that simulates a transmission exposure of the acquisition region. For a uniform scatter and attenuation correction, the anatomical component of the computer model is segmented into tissue types, with which attenuation coefficients are in turn associated.
Furthermore, a pathology-related NM atlas can be used that, for example, is indexed to the clinical appearance [pathological pattern] of a stroke. Pathology-dependent changes of the anatomy are thereby accounted for in the NM atlas. Such a computer model also can be generated by segmentation of an MR atlas image, meaning that, for example, a SPECT atlas image and an anatomical atlas image are generated from the MR atlas image. For example, the former can correspond to a SPECT measurement and the latter to a transmission measurement.
An example for an MR atlas is disclosed in United States Patent Application Publication Ser. No. 2003/0139659. The MR atlas contains representative values of MR properties of an MR examination of a “reference patient,” and optionally contains tissue-specific probabilities. The MR atlas can be used, for example, together with a test measurement in order to establish a specific geometry of slices of an MR measurement to be measured. For example, using the MR atlas a section guide (slice guide) generated using an MR atlas image for each patient can be transferred onto an MR exposure of a patient. For this purpose, a rotation and translation transformation that maps the test MR image to the MR atlas image is determined from a comparison of a test MR image with the MR atlas image. The position of the patient is thereby known relative to the reference patient and, by means of the rotation, dilation, compression and translation transformation, predefined standard sections can be automatically transferred from the geometry of the atlas/reference patient to the patient in the MR apparatus and MR data then can be suitably acquired.
As noted, NM data can be processed into an NM image with an NM atlas for scatter and attenuation correction. One difficulty in the use of such an NM atlas is the low image quality of, for example, functional PET images in which, for example, only the function of a small region in the brain is shown. A registration of, for example, a PET image with a PET atlas image thus has inaccuracies associated therewith.
The acquisition of, for example, CT or MR images, as well as nuclear-medical images, is possible with a system known as a dual modality tomography apparatus. In combination with a CT apparatus, the required information about scatter and attenuation coefficients of the examined tissue can be directly acquired from the x-ray exposures. This means that the test measurement is omitted due to the ability to register the NM and CT measurements and to obtain the required information directly from the CT data. This is not directly possible in a combination of NM apparatus and MR apparatus. In particular, tissue differentiation between bones, lungs and soft anatomy (of which bones are not directly imaged in the MR image) is necessary for the determination of the attenuation coefficients.
The required information about the curvature of bones, however, can be acquired by manual or automatic segmentation. For example, for this purpose a patient is examined with a number of specially-parameterized MR measurement sequences that, for example, makes use of different parameters such as T1 or T2. MR images thus are acquired in which various tissue types are shown differently. The tissue types then can be classified with a segmentation algorithm and provided with attenuation coefficients. An attenuation correction matrix is subsequently created for the examined region. Zaldi et al. describe such a procedure in “Magnetic resonance imaging-guided attenuation and scatter corrections in three dimensional brain position emission tomography”, Medical Physics, Vol. 30 (#5), May 2003, p. 937-947.
A method for superimposition of PET and MR brain images is known from Jesper et al.: “A Method of Coregistration of PET and MR brain images”, (1995) The Journal of Nuclear Medicine, Vol. 36, No. 7, p. 1307-1315. A simulated PET image is thereby generated from an MR image via segmentation and by association of “acquisition” values. A transformation that maps the two images one over the other is determined using the comparison of the simulated PET image with the measured PET image. This transformation is subsequently applied to the superimposition of MR image and PET image.